Reaction of an optically active (1-methyl-2,2-diphenylcyclopropyl)trimethyltin (I) with bromine (or iodine) afforded 1-bromo-(or iodo)-1-methyl-2,2-diphenylcyclopropane (II) with a small degree of retention of configuration. This is best interpreted in terms of radical mechanism for the cleavage of the cyclopropyl carbontin bond in (I) by bromine or iodine.